Integrated fan drive system for cooling tower

ABSTRACT

An integrated fan drive system for a cooling tower comprising a high-torque, low speed permanent magnet motor having a rotatable shaft, a fan comprising a hub that is directly connected to the rotatable shaft and a plurality of fan blades that are attached to the hub, and a variable frequency drive device in electrical signal communication with the permanent magnet motor to control the rotational speed of the permanent magnet motor. The high-torque, permanent magnet motor comprises no more than two bearings in operative association with the shaft. The variable frequency drive device has a variable frequency controller that has an input for receiving AC power and an output for providing electrical signals that control the operational speed of high-torque, permanent magnet motor. The variable frequency drive device also includes a user interface in electronic data signal communication with the variable frequency controller to allow a user to input motor speed control data.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/978,916, filed Oct. 10, 2007. The entire disclosure of the aforesaidapplication No. 60/978,916 is hereby incorporated by reference.

TECHNICAL FIELD

The present invention generally relates to a fan drive system for a wetcooling tower.

BACKGROUND ART

Wet cooling towers are well known in the art and are used in a varietyof industries for cooling fluids such as water. The primary use oflarge, industrial cooling tower systems is to remove the heat absorbedin circulating cooling water systems used in power plants, petroleumrefineries, petrochemical and chemical plants, natural gas processingplants and other industrial facilities. The absorbed heat is rejected tothe atmosphere by the evaporation of some of the cooling water inmechanical forced-draft or induced draft towers.

Cooling towers are widely used in the petroleum refining industry.Refining of petroleum cannot take place without cooling towers.Refineries process hydrocarbons at high temperatures and pressures.Cooling water is used to control operating temperatures and pressures.The loss of cooling water circulation within a refinery can lead tounstable and dangerous operating conditions requiring an immediate shutdown of processing units. Cooling towers have become “mission criticalassets” for petroleum refinery production. As demand for high-endproducts such as automotive and aviation fuel has risen and refiningcapacity has shrunk, the refineries have incorporated many new processesthat extract hydrogen from the lower value by-products and recombinedthem into the higher value fuels, improving yield. Many of theseprocesses are dependant on cooling to optimize the yield and quality ofthe product. Over the past decade, many refineries have been addingprocesses that reform low grade petroleum products into higher grade andmore profitable products such as aviation and automotive gasoline. Theseprocesses are highly dependent upon the cooling towers to control theprocess temperatures and pressures that affect the product quality,process yield and safety of the process. In addition, these processeshave tapped a great deal of the cooling capacity reserve in the towersleaving some refineries “cooling limited” on hot days and evenbottlenecked. With most U.S. refineries operating well above 90%capacity with attractive profit margins, operating the refinery iscritical to operating profit and to pay for the process upgradesimplemented over the last decade.

Typically, a wet cooling tower system comprises a basin which holdscooling water that is routed through the process coolers and condensersin an industrial facility. The cool water absorbs heat from the hotprocess streams that need to be cooled or condensed, and the absorbedheat warms the circulating water. The warm circulating water isdelivered to the top of the cooling tower and trickles downward overfill material inside the tower. The fill material is configured toprovide a maximum contact surface, and maximum contact time, between thewater and air. As the water trickles downward over the fill material, itcontacts ambient air rising up through the tower either by natural draftor by forced draft using large fans in the tower. Many wet coolingtowers comprise a plurality of cells in which the cooling of water takesplace in each cell in accordance with the foregoing technique. Coolingtowers are described extensively in the treatise entitled “Cooling TowerFundamentals”, second edition, 2006, edited by John C. Hensley,published by SPX Cooling Technologies, Inc.

Many cooling towers in use today utilize large fans, as described in theforegoing discussion, to provide the ambient air. The fans are enclosedwithin a fan cylinder that is located on the fan deck of the coolingtower. Drive systems are used to drive and rotate the fans. Theefficiency and production rate of a cooling tower is heavily dependentupon the efficiency of the fan drive system. The duty cycle required ofthe fan drive system in a cooling tower environment is extreme due tointense humidity, icing conditions, wind shear forces, corrosive watertreatment chemicals, and demanding mechanical drive requirements.

One commonly used prior art drive system is a complex, mechanical fandrive system that is similar to the type used in agricultureapplications. This type of prior art fan drive system utilizes a motorthat drives a drive train. The drive train is coupled to a gearbox,gear-reducer or speed-reducer which is coupled to and drives the fan.This prior art fan drive system is subject to frequent outages, aless-than-desirable MTBF (Mean Time Between Failure), and requiresdiligent maintenance, such as regular oil changes, in order to operateeffectively. Furthermore, prior art gearboxes typically require aseparate gear to reverse the rotational direction. One common type ofmechanical drive system used in the prior art gearbox-type fan driveutilizes five rotating shafts, eight bearings, three shaft seals (two athigh speed), and four gears (two meshes). This drive train absorbs about3% of the total power. Although this particular prior art fan drivesystem may have an attractive initial cost, cooling tower end-usersfound it necessary to purchase additional components such as compositegearbox shafts and couplings in order to prevent breakage of the fandrive components. Many cooling tower end-users also added other optionssuch as low-oil shutdown, anti-reverse clutches and oil bath heaters.Thus, the life cycle cost of the prior art mechanical fan drive systemcompared to its initial purchase price is not equitable.

In a multi-cell cooling tower, such as the type commonly used in thepetroleum industry, there is a fan and prior art mechanical fan drivesystem associated with each cell. Thus, if there is a shutdown of themechanical fan drive system associated with a particular cell, then thatcell suffers a “cell outage”. A cell outage will result in a decrease inthe production of refined petroleum. For example, a “cell outage”lasting for only one day can result in the loss of thousands of refinedbarrels of petroleum. If numerous cells experience outages lasting morethan one day, the production efficiency of the refinery can besignificantly degraded. The loss in productivity over a period of timedue to the inefficiency of the prior art mechanical fan drive systemscan be measured as a percent loss in total tower-cooling potential. Asmore cell outages occur within a given time frame, the percent loss intotal tower-cooling potential will increase. This, in turn, willdecrease product output and profitability of the refinery and cause anincrease in the cost of the refined product to the end user. It is notuncommon for decreases in the output of petroleum refineries, even ifslight, to cause an increase in the price-per-barrel of petroleum andhence, an increase in the cost of gasoline to consumers. The effect ofcell outages with respect to the impact of petroleum product prices isdescribed in the report entitled “Refinery Outages: Description andPotential Impact On Petroleum Product Prices”, March 2007, U.S.Department of Energy.

Other types of prior art fan drive systems, such as V-belt drivesystems, also exhibit many problems with respect to maintenance, MTBFand performance and do not overcome or eliminate the problems associatedwith the prior art gearbox-type fan drive systems. One attempt toeliminate the problems associated with the prior art gearbox-type fandrive system is the prior art hydraulically driven fan system. Such asystem is described in U.S. Pat. No. 4,955,585 entitled “HydraulicallyDriven Fan System For Water Cooling Tower”.

Therefore, in order to prevent supply interruption of the inelasticsupply chain of refined petroleum products, the reliability andsubsequent performance of cooling towers must be improved and managed asa key asset to refinery production and profit. An efficient and reliablefan drive system is required to maintain a relatively high coolingefficiency and prevent interruptions in production.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to provide a fandrive system that substantially eliminates the aforementioned problemsand disadvantages associated with prior art gearbox-type fan drivesystems.

Thus, present invention is directed to, in one aspect, a fan drivesystem comprising a high-torque, low speed, permanent magnet motorhaving a rotatable shaft, a fan comprising a hub that is directlyconnected to the rotatable shaft and a plurality of fan blades that areattached to the hub, and a variable frequency drive device in electricalsignal communication with the permanent magnet motor to control therotational speed of the permanent magnet motor. The variable frequencydrive device has a variable frequency controller that has an input forreceiving AC power and an output for providing electrical signals thatcontrol the operational speed of the high-torque, low speed permanentmagnet motor. The variable frequency drive device also includes a userinterface in electronic data signal communication with the variablefrequency controller to allow a user to input motor speed control data.

In a related aspect, the present invention is directed to thecombination of a wet cooling tower having a fan deck, a fan cylinderpositioned upon the fan deck, and a fan located within the fan cylinder.The fan comprises a hub to which are connected a plurality of fanblades. The combination further includes a high-torque, permanent magnetmotor that has a rotatable shaft connected to the hub, and a variablefrequency drive device in electrical signal communication with thepermanent magnet motor to control the rotational speed of the permanentmagnet motor. The variable frequency drive device comprises a variablefrequency controller that has an input for receiving AC power and anoutput for providing electrical signals that control the operationalspeed of high-torque, permanent magnet motor, and a user interface inelectronic data signal communication with the variable frequencycontroller to allow a user to input data representing desired motorspeeds. The combination further comprises a plurality of sensors inelectronic data signal communication with the user interface to providesensor data signals representing vibration and heat at the bearings ofthe high-torque, permanent magnet motor, heat of the motor stator, andair flow created by rotation of the fan.

Other objects of the present invention, as well as particular features,elements and advantages thereof, will be elucidated in, or be apparentfrom, the following description and the accompanying drawing figures.

BRIEF DESCRIPTION OF THE DRAWINGS

Understanding of the present invention and the various aspects thereofwill be facilitated by reference to the accompanying drawing figures,submitted for the purposes of illustration only and not intended todefine the scope of the invention, in which:

FIG. 1 is an elevational view, partially in cross-section, of a fancylinder supported by a fan deck of a cooling tower, a fan within thefan cylinder and a prior art gearbox-type fan drive system;

FIG. 2A is an elevational view, partially in cross-section, of a fancylinder supported by a fan deck of a cooling tower, a fan within thefan cylinder and the fan drive system of the present invention;

FIG. 2B is a plot of motor speed versus horsepower for a high torque,low speed permanent magnet motor used in one embodiment of the fan drivesystem of the present invention;

FIG. 3 is a block diagram of the fan drive system of the presentinvention;

FIG. 4 is a graph illustrating a comparison in performance between thefan drive system of the present invention and a prior art gearbox-typefan drive system that uses a variable speed induction motor; and

FIG. 5 is a schematic diagram showing the fan drive system of thepresent invention in conjunction with a plurality of performancemonitoring sensors.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, there is shown a prior art mechanical fan drivesystem, and a portion of a wet cooling tower. The remaining portion ofthe wet cooling tower is not shown since the structure and operation ofwet cooling towers is well known in the art. Fan cylinder 10 ispositioned on fan deck 12 of the cooling tower. The prior art mechanicalfan drive system comprises induction motor 14, drive shaft 16, couplings18 and 20, and right-angle gearbox 22. Motor 14 is seated on and/orsecured to fan deck 12. Gearbox or gear reducer 22 is mounted to orsupported by fan deck 12. Gearbox 22 has a vertical shaft 24 thatrotates upon rotation of drive shaft 16. As shown in FIG. 1, fan 27 islocated within fan cylinder 10 and comprises hub 28 and fan blades 30that are attached to hub 28. Vertical shaft 24 is connected to fan hub28. Thus, rotation of vertical shaft 24 causes rotation of fan hub 28and fan blades 30.

Referring to FIG. 2A, there is shown the fan drive system of the presentinvention. Similar to the view of FIG. 1, a portion of the cooling toweris only shown in FIG. 2A. The fan drive system of the present inventioncomprises variable frequency drive (VFD) device 50 and motor 52. Inaccordance with the invention, motor 52 is a high torque, low speedpermanent magnet motor. Permanent magnet motor 52 has a high fluxdensity. The superior results, advantages and benefits resulting frompermanent magnet motor 52 are discussed in the ensuing description. VFDdevice 50 and permanent magnet motor 52 are mounted to or supported byfan deck 12. VFD device 50 is in electrical signal communication withpermanent magnet motor 52 via cables or wires 54. Permanent magnet motor52 has shaft 56 that rotates when the appropriate electrical signals areapplied to permanent magnet motor 52. Shaft 56 is connected to fan hub28. Thus, rotation of vertical shaft 56 causes rotation of fan hub 28and fan blades 30.

Referring to FIGS. 2A and 3, VFD device 50 comprises a variablefrequency controller 60 and a user or operator interface 62. VFD device50 controls the speed, direction (i.e. clockwise or counterclockwise),and torque of permanent magnet motor 52. AC input power is inputted intovariable frequency controller 60 via input 64. Variable frequencycontroller 60 converts the AC input power to DC intermediate power.Variable frequency controller 60 then converts the DC power intoquasi-sinusoidal AC power that is applied to permanent magnet motor 52.User interface 62 provides a means for an operator to start and stoppermanent magnet motor 52 and adjust the operating speed of motor 52. Ina preferred embodiment, user interface 62 comprises a microprocessor,and an alphanumeric display and/or indication lights and meters toprovide information about the operation of motor 52. User interface 62further includes a keypad and keypad display that allows the user toinput desired motor operating speeds. VFD device 50 includes input andoutput terminals 70 and 72 for connecting pushbuttons, switches andother operator interface devices or controls signals. In a preferredembodiment, VFD device 50 further includes a serial data communicationport 80 to allow VFD device 50 to be configured, adjusted, monitored andcontrolled using a computer. In one embodiment, VFD device 50 includessensor signal inputs 82, 84, 86, 88 and 89 for receiving sensor outputsignals. The purpose of these sensors is discussed in the ensuingdescription.

Referring to FIGS. 2A and 5, permanent magnet motor 52 is directlycoupled to the fan hub 28. Since permanent magnet motor 52 is controlledonly by electrical signals provided by VFD device 50, there is no driveshaft, couplings, gear boxes or related components as is found in theprior art gearbox-type fan drive system shown in FIG. 1. In accordancewith the invention, permanent magnet motor 52 is a high-torque, lowspeed motor. Permanent magnet motor 52 is of simplified design and usesonly two bearings 90 and 92 (see FIG. 5). Permanent magnet motor 52includes stator 94. Such a simplified design provides relatively highreliability as well as improved and cost-effective motor production.Permanent magnet motor 52 has relatively low maintenance with a threeyear lube interval. Permanent magnet motor 52 can be configured withsealed bearings. Permanent magnet motor 52 meets or exceeds theefficiency of Premium Efficiency Induction Motors. Permanent magnetmotor 52 substantially reduces the man-hours associated with service andmaintenance that would normally be required with a prior art, inductionmotor fan drive system. In some instances, permanent magnet motor 52 caneliminate up to 1000 man-hours of maintenance and service. Suchreliability reduces the amount of cell outages and significantlyimproves product output. In one embodiment, permanent magnet motor 52has the following operational and performance characteristics:

Speed Range: 0-250 RPM

Maximum Power: 133 HP/100 KW

Number of Poles: 16

Motor Service Factor: 1:1

Rated Current: 62 A (rms)

Peak Current: 95 A

Rated Voltage: 600 V

Drive Inputs: 460 V, 3 phase, 60 Hz, 95 A (rms max. continuous) FIG. 2Bshows a plot of speed vs. horsepower for high torque, low speedpermanent magnet motor 52. However, it is to be understood that theaforesaid operational and performance characteristics just pertain toone embodiment of permanent magnet motor 52 and that motor 52 may bemodified to provide other operational and performance characteristicsthat are suited to a particular application.

Referring to FIG. 4, there is shown a graph that shows “Efficiency %”versus “Motor Speed (RPM)” for the fan drive system of the presentinvention and a prior art fan drive system using a variable speed,induction motor. Curve 100 pertains to the present invention and curve102 pertains to the aforementioned prior art fan drive system. As can beseen in the graph, the efficiency of the present invention is relativelyhigher than the prior art fan drive system for motor speeds betweenabout 60 RPM and about 200 RPM.

Referring to FIG. 5, in a preferred embodiment, the fan drive system ofthe present invention further comprises a plurality of sensors 200, 202,204, 206 and 208 that provide sensor signals to sensor signal inputs 82,84, 86, 88 and 89, respectively, of VFD device 50. Sensors 200 and 202are positioned in proximity to bearings 90 and 92, respectively, ofpermanent magnet motor 52 in order to sense vibration and heat. Sensor204 is positioned on stator 94 of permanent magnet motor 52 to monitorheat at stator 94. Sensor 206 is positioned down stream of the air flowcreated by fan 27 to measure airflow. For purposes of simplifying FIG.5, fan 27 is not shown. Sensor 208 is located within the basin (notshown) of the wet cooling tower to sense the temperature of the waterwithin the basin. All sensor output signals applied to sensor signalinputs 82, 84, 86, 88 and 89 are inputted into user interface 62 of VFDdevice 50 and are then routed to an external processing device, such ascomputer 300, via data port 80. Computer 300 includes a display screendevice 302 that enables a user or operator to visually monitor the dataoutputted by sensors 200, 202, 204, 206 and 208. Computer 300 furtherincludes a user interface, e.g. keyboard, (not shown) that allows anoperator to input commands. Computer 300 is configured to implement areliability algorithm using the data outputted by sensors 200, 202, 204,206 and 208 and in response, output appropriate control signals that areinputted into user interface 62 via data port 80. Such control signalscan be used to adjust the speed of motor 52. Thus, the sensors andcomputer 300 provide a feedback loop that:

-   -   a) monitors vibrations and heat at the bearings of motor 52;    -   b) monitors heat at the stator of motor 52;    -   c) monitors airflow produced by fan 27;    -   d) monitors the temperature of the water in the cooling tower        basin;    -   e) provides a trim balance to compensate for fan-unbalance        inertia on the cooling tower structure (i.e. “Hula Effect”);    -   f) alerts the operators to a “blade-out” situation and        automatically reduces the speed of motor 52;    -   g) locks out a particular motor speed that creates resonance;    -   h) alerts the operator to ice accumulation on fan blades 30 and        automatically initiates de-icing operations; and    -   i) routes the basin-water temperature data to other portions of        the industrial process so as to provide real-time cooling        feedback information that can be used to make other adjustments        in the overall industrial process.

Thus, the fan drive system of the present invention provides manyadvantages and benefits, including:

-   -   a) elimination of many components found in the prior art        gearbox-type fan drives, such as drive shafts, couplings,        bearings, shaft seals, etc.;    -   b) elimination of oil changes;    -   c) significant reduction in service and maintenance;    -   d) ability to vary the speed of the permanent magnet motor over        a relative wide range of speeds;    -   e) ability to reverse direction of the permanent magnet motor        without any additional components;    -   f) consumption of significantly lower amounts of energy in        comparison to prior art gearbox-type fan drive;    -   g) easy retrofit with existing fan thereby eliminating need to        construct new cooling towers;    -   h) significant reduction in the occurrence of cell outages; and    -   i) provides significantly more cooling capacity in comparison to        prior art gearbox-type fan drive.

The operational logic and system architecture of the present inventionwill provide the ability to optimize the cooling tower for energyefficiency (e.g. at night when it is cold) and to maximize cooling onhot days or when the process demands additional cooling or to avoidfouling of auxiliary systems such as condenser and heat exchangers.

Although the foregoing discussion is in terms of the applicability ofthe present invention to the petroleum industry, it is to be understoodthat the present invention provides benefits to any industry that useswet cooling towers. Thus, the present invention has applicability tomany industries that consume large amounts of energy and are processintensive, such as the power generation, petro-chemical, pulp and paper,chemical, glass, mining, steel and aluminum industries.

It will thus be seen that the objects set forth above, among thoseelucidated in, or made apparent from, the preceding description, areefficiently attained and, since certain changes may be made in the aboveconstruction and/or method without departing from the scope of theinvention, it is intended that all matter contained in the abovedescription or shown in the accompanying drawing figures shall beinterpreted as illustrative only and not in a limiting sense. It is alsoto be understood that the following claims are intended to cover all ofthe generic and specific features of the invention herein described.

1. A drive system for driving a fan in a cooling tower, the fan comprising a fan hub and fan blades attached to the fan hub, the drive system comprising: a high-torque, low speed permanent magnet motor comprising a motor casing, a stator and a rotatable shaft, the rotatable shaft being configured for connection to the fan hub, the motor further comprising a dual bearing system consisting of a pair of radial bearings that locate and support the rotatable shaft relative to the motor casing; and a variable frequency drive device to generate electrical signals that effect rotation of the rotatable shaft of the motor at a rotational speed in order to rotate the fan.
 2. The drive system according to claim 1 wherein the variable frequency drive device comprises electronic circuitry for receiving control signals that represent a particular rotational speed and generating electrical signals for the permanent magnet motor, wherein the electrical signals represent the rotational speed represented by the received control signals.
 3. The drive system according to claim 2 further comprising: vibration sensors located within the motor casing to measure vibrations at the radial bearings and output signals representing the measured vibrations; and a plurality of heat sensors located within the motor casing to measure heat at the stator and at the radial bearings, the heat sensors outputting signals representing the measured heat.
 4. The drive system according to claim 3 further comprising a processor for generating speed control signals for input into the electronic circuitry of the variable frequency drive device, wherein the speed control signals vary the speed of the high-torque, low speed permanent magnet motor, the processor being configured to process the signals outputted by the vibration and heat sensors to determine vibration level and stator heat, respectively.
 5. A method of installing a new fan drive system in a wet-cooling tower having a fan assembly that comprises a fan hub and a plurality of fan blades that are attached to the fan hub, the wet-cooling tower also having a preexisting fan drive system for driving the fan assembly, wherein the preexisting fan drive system has a gearbox, a drive shaft that drives the gearbox and an induction motor that drives the drive shaft, the method comprising: disconnecting the gearbox from the fan hub and removing the gearbox; removing the drive shaft; removing the induction motor; providing a high-torque, low speed permanent magnet motor comprising a motor casing, a stator and a rotatable shaft, the motor further comprising a dual bearing system consisting of a pair of radial bearings that locate and support the rotatable shaft relative to the motor casing; and connecting the rotatable shaft of the motor to the fan hub.
 6. The method according to claim 5 wherein the permanent magnet motor has a plurality of heat sensors located within the motor casing to measure heat at the stator and heat at the radial bearings.
 7. The method according to claim 6 wherein the permanent magnet motor includes vibration sensors located within the motor casing to measure the level of vibrations at the radial bearings.
 8. The method according to claim 5 further comprising the steps: providing a variable frequency drive device to power the permanent magnet motor; and electrically connecting the variable frequency drive device to the permanent magnet motor.
 9. A wet-cooling tower comprising: a fan deck; a fan cylinder positioned upon the fan deck; a fan assembly located within the fan cylinder, the fan assembly comprising a fan hub and a plurality of fan blades connected to the fan hub; a basin for receiving water; a high-torque, low speed permanent magnet motor comprising a motor casing, a stator supported by the casing and a rotatable shaft, the rotatable shaft being connected to the fan hub, the motor further comprising a dual bearing system consisting of a pair of radial bearings that locate and support the rotatable shaft relative to the motor casing; a plurality of heat sensors located within the motor casing to measure heat at the stator and heat at the radial bearings and to output signals that represent the measured heat; vibration sensors located within the motor casing to measure the level of vibrations at the radial bearings and output signals that represent the level of vibrations; an air-flow sensor to measure the air-flow generated by the fan assembly and output signals that represent the level of air-flow generated by the fan assembly; a temperature sensor in the basin to measure the temperature of the water in the basin; a variable frequency drive device in electrical signal communication with the permanent magnet motor to control the rotational speed of the permanent magnet motor, the variable frequency drive device comprising electronic circuitry for receiving control signals that represent a particular motor rotational speed and generating electrical signals for the permanent magnet motor that correspond to the received control signals; and a processor to process the signals outputted by the vibration, heat, air-flow and temperature sensors and generate control signals for input into the electronic circuitry of the variable frequency drive device.
 10. A cooling tower, comprising: a fan assembly comprising a fan hub and fan blades attached to the fan hub; a high-torque, low speed permanent magnet motor comprising a motor casing, a stator assembly supported by the motor casing and a rotatable shaft, the rotatable shaft being connected to the fan hub, the motor further comprising a dual bearing system consisting of a pair of radial bearings that locate and support the rotatable shaft relative to the motor casing; and a variable frequency drive device to generate electrical signals that effect rotation of the rotatable shaft of the motor at a rotational speed so as to rotate the fan.
 11. A wet-cooling tower comprising: a fan assembly comprising a fan hub and a plurality of fan blades connected to the fan hub; a high-torque, low speed permanent magnet motor comprising a motor casing, a stator attached to the motor casing and a rotatable shaft, the rotatable shaft being connected to the fan hub, the motor further comprising a dual bearing system consisting of a pair of radial bearings that locate and support the rotatable shaft relative to the motor casing; a plurality of heat sensors located within the motor casing to measure heat at the stator and heat at the radial bearings and to output signals that represent the measured heat; vibration sensors located within the motor casing to measure the level of vibrations at the radial bearings, the vibration sensors outputting signals that represent the level of vibrations; an air-flow sensor to measure the air-flow generated by the fan assembly and output signals that represent the level of air-flow generated by the fan assembly; a variable frequency drive device in electrical signal communication with the permanent magnet motor to control the rotational speed of the permanent magnet motor, the variable frequency drive device comprising electronic circuitry for receiving control signals that represent a particular motor rotational speed and generating electrical signals for the permanent magnet motor that correspond to the received control signals; and a processor for processing the signals outputted by the vibration, heat and air-flow sensors and generating control signals for input into the electronic circuitry of the variable frequency drive device. 